Electronic circuits contain many (sometimes millions) of components such as resistors, capacitors, inductors, diodes, electromechanical switches, and transistors. High density packaging of electronic components is particularly important to allow fast access to large amounts of data in computers. High density electronic circuit packages also are important in high frequency devices and communications devices. The components are connected to form circuits and circuits are connected to form functioning devices. The connections perform power and signal distribution. In a multi-layer electronic circuit package, some layers of the package serve as power planes and other layers serve as signal planes, depending on the operational requirements of the device. The devices require mechanical support and structural protection. The circuits themselves require electrical energy to function. The functioning devices, however, produce heat, or thermal energy which must be dissipated so that the devices do not stop functioning. Moreover, while high density packaging of a number of components can improve performance of the device, the heat produced by the power-consuming components can be such that performance and reliability of the devices is adversely impacted. The adverse impact arises from electrical problems such as increased resistivity and mechanical problems such as thermal stress caused by increased heat.
A further heat-related issue is that the heat will cause the components and the electronic circuit package to expand. Where there is a difference in the coefficient of thermal expansion of the components compared to the electronic circuit package material (as there usually is), the solder joint between each component and the electronic circuit package will be stressed.
High density packages necessarily involve increased wiring density and thinner dielectric coatings between layers in a multi-layer electronic circuit package. The layers in a multi-layer package are electrically connected by vias and through-holes. The term "via" is used for a conductive pathway between adjacent layers in a multi-layer electronic circuit package. The term "through-hole" is used for a conductive pathway that extends to a non-adjacent layer. For high density packages the through-holes are increasingly narrow in diameter and the through-holes in each layer must be aligned precisely.
Electronic circuit packages, such as chips, modules, circuit cards, circuit boards, and combinations of these, thus must meet a number of requirements for optimum performance. The package must be structurally sturdy enough to support and protect the components and the wiring. In addition, the package must be capable of dissipating heat and must have a coefficient of thermal expansion that is compatible with that of the components. Finally, to be commercially useful, the package should be inexpensive to produce and easy to manufacture.
The prior art contains many examples of electronic circuit packages. These packages can be generally classified into packages with either a ceramic dielectric coating between layers or those with a polymeric dielectric coating between layers. One type of polymeric package is a metal core package--namely a package with aluminum, copper, molylbdenum, or copper-Invar-copper as the core, which is then encapsulated in a polymeric dielectric. Metal cores are particularly attractive since they are useful in dissipating the heat generated by the components and wiring mounted on the board. Metal cores also have the mechanical strength needed to hold the components. For a discussion of the aluminum metal core, see Belke, et al., U.S. Pat. No. 4,679,122. The copper-Invar-copper metal core is particularly widely used in the industry. Invar is a registered trademark of Imphy S.A. of Paris, France for an iron/nickel alloy that is 64% iron by weight and 36% nickel by weight.
There are several disadvantages, however, associated with the copper-Invar-copper metal core. These include first, that exposed Invar at the edges and corners of the package is susceptible to corrosion which can cause manufacturing problems and problems once the electronic circuit package is in use. Second, the copper-Invar-copper core has a sandwich-like construction which suffers from adhesion and delamination problems. Third, the copper-Invar-copper core is expensive to produce due to the various metallurgical operations needed to produce it. Fourth, the copper-Invar-copper core is not widely manufactured and thus is priced higher than would be a material available from a number of sources, which increases the manufacturing cost for electronic circuit packages with a copper-Invar-copper metal core.